I have it! The way to stop people making the illegal turn off Honor Oak Road into Manor Mount is simple. Make the phase of the traffic lights turning out of Honor Oak Road onto Lordship Lane longer than just two milliseconds, thereby eliminating the huge queue up the hill and, at a stroke, the temptation to illegally run down Manor Mount. That's the real root cause of the problem that should be tackled, rather than the symptoms.

It would be so simple. However, I bet TfL would never agree to it, as it might eff up their precious red route, leaving the council, who is responsible for the back roads, to deal with the consequences.